@pa_fog_man I think we have a bit of confusion here (and the 390 is not helping with clarification). First let me say that the Dell’s will let you dynamically boot in the other mode without changing the firmware (where it gets its default value). You can do this dynamic switching from the F12 boot menu. The other issue with the 390s (I can’t speak for personal experience with the 390s, but we have 790s in our fleet) the 790s were the first generation to support UEFI mode. But as far as I know, the 790s don’t support pxe booting in uefi mode. The 7010s was the first generation to support pxe booting in uefi mode. To pxe boot on the 7010s you would have to enable the uefi network stack on the network adapter settings in the bios. Otherwise you would not get ipv4 and ipv6 entries under uefi on the F12 boot menu.
Back on point. You MUST select the proper boot kernel for the target system bios mode (legacy/bios or UEFI). If you sent the wrong boot file to the system you will not see the FOG iPXE boot menu.
for bios mode you can use ipxe.pxe or undionly.kpxe
for uefi mode you can use ipxe.efi or snponly.efi
The bold text above are the typical kernels to set for dhcp option 67.
You can not mix the boot kernels. This isn’t a FOG issue, but rather a firmware one.